In constructing fixed digital radio link connections, it is essential that the probability of the connection being broken remains sufficiently low. In addition to equipment reliability, this probability is dependent on phenomena in the radio path affecting the radio wave propagation. Such phenomena include:                1. normal free-space attenuation,        2. attenuation caused by rain, slowly changing in frequency and time,        3. splitting of the signal propagation path, due to changes in the atmospheric refractive index or reflections caused by terrain obstructions, into two or more routes (multipath propagation), and frequency-selective attenuation caused by the interaction of said routes, and        4. interference arriving at the receiver from other radio link hops or other equipment.        
Consequently, the transmission power of the transmitter should be temporarily increased so that additional signal attenuation on account of phenomena 2 and 3 alone or in combination are not capable of excessively degrading the quality of the signal received. However, high transmission power increases, on the other hand, interference in adjacent channels and radio hops. This effect from phenomenon 4 of the increased interference brings about deterioration of the sensitivity of other receivers. Furthermore, high transmission power hinders the implementation of dense radio link networks and efficient utilization of the frequencies available for radio link systems.
The task of an automatic power control in a radio link system is to continuously adjust the transmission power of the transmitter according to the changes affecting the quality of the signal received so that error-free reception can be ensured as well as possible.
The power control mechanism has conventionally been implemented by making a signal level measurement on the received signal at the far end of the radio hop. Then a signal representing the measured signal level is sent through a return channel to the transmitter and the transmission power is controlled so that the level of the received signal remains substantially constant at the receiving end.
A major drawback of this kind of power control system is that it is not capable of taking into account the effects of external interferences (phenomenons 3 and 4) on the signal quality.
Some other known power control mechanisms are based on bit error measurement at the receiving end of the radio hop. In response to this measurement, the transmission power at the opposite end is then controlled in such a way that a predetermined error threshold is not exceeded at the receiving end. In this way, the effect of external interference can be taken into account in the power control, at least to some extent.
In addition to bit error measurement, the power control mechanism can also be based on other measurements made at the receiving end. A more sophisticated power control mechanism such as this is described in EP-B1-0428099. In this power control mechanism, the bit error rate and the rate of change of the received signal level are estimated by the receiver. If either the error rate estimate or, alternatively, the rate of change in the received signal level exceeds a respective predetermined threshold value, the transmission power is temporarily increased to a value appreciably higher than the normal transmission power but lower than the maximum transmission power. On the other hand, should both the error rate estimate and the rate of change in the received signal level simultaneously exceed the respective predetermined threshold value, the transmission power is increased to its maximum value for a predetermined time. This is done independently of the level of the received signal. After the predetermined time has elapsed, the transmitter power is gradually decreased until a certain signal level is reached, until bit errors appear again or until the rate of change of the received signal level exceeds the respective threshold value.
The drawback relating to the above known power control mechanisms based on bit error monitoring is that bit errors degrade the signal quality before the transmitter power can be increased. Furthermore. in systems such as the one described in EP-B1-0428099, the high or full transmission power may often be more than what is actually needed for error-free reception. Thus unnecessary interference may be generated for adjacent radio channels or systems.